Epithermal Deposits Related to Caldera Development in Newly- Identified Graben, Oaxaca, Mexico

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Nathaniel Small
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1 Rocky Mountain Section 68th Annual Meeting Epithermal Deposits Related to Caldera Development in Newly- Identified Graben, Oaxaca, Mexico BARRY D. DEVLIN Vice President, Exploration May 18,

2 Mexico Epithermal Deposits (Camprubi & Albinson, 2007) OAXACA 2

3 Mexico Geology Sierra Madre Del Sur Sierra Madre Del Sur 3

More than 8,000 earthquakes with magnitude greater than 4.0 in 40 years.

4 Seismotectonics of Southern Mexico Tertiary volcanism & ore deposits are superimposed on composite thrustbound basement terranes associated with evolution of NW-trending oblique right-lateral convergent Cocos-North American plate margin. More than 8,000 earthquakes with magnitude greater than 4.0 in 40 years. Seismicity from 1973 to present USGS National Earthquake Information Center (NEIC) database) Seismic Hazard Study El Aguila Project by Vector Engineering Inc. (after Kostoglodov & Ponce, 1994) 4

Tectonostratigraphic Terranes of Oaxaca OAXACA BASIN AGUILA MINE Cuicateco Terrane consisting of Mesozoic volcanosedimentary sequence formed in a submarine volcanic arc with a

5 Tectonostratigraphic Terranes of Oaxaca OAXACA BASIN AGUILA MINE Cuicateco Terrane consisting of Mesozoic volcanosedimentary sequence formed in a submarine volcanic arc with a complex metamorphic basement, and Zapoteco terrane consisting of Proterozoic continental crust of mainly crystalline basement rocks overlain nonconformably by rare Paleozoic strata. 5

Oaxaca Geology - Lithostratigraphy Modified from Geology Map of Oaxaca by Servicio Geologico Mexicano (SGM) Tertiary volcanics dominated by rhyolitic to andesitic

6 Oaxaca Geology - Lithostratigraphy Modified from Geology Map of Oaxaca by Servicio Geologico Mexicano (SGM) Tertiary volcanics dominated by rhyolitic to andesitic tuffs, minor flows, agglomerates & ignimbrites Generally accepted potassium-argon dates are Miocene Primarily underlain by Cretaceous limestones, sandstones and siltstones 6

Miocene Taviche Graben SW-NE directed contraction in Paleocene to Oligocene followed by transtensional, left-lateral displacement on major faults In the Miocene, this changed to NE-SW directed

7 Miocene Taviche Graben SW-NE directed contraction in Paleocene to Oligocene followed by transtensional, left-lateral displacement on major faults In the Miocene, this changed to NE-SW directed extension with normal faults Structural Trough 200 km long & 75 km wide on high-angle faults preserves thick section of Miocene volcanic rocks OAXACA BASIN NATIVIDAD AGUILA SAN JOSE TAVICHE GRABEN 7

flow domes overlain by thick ignimbrite units are associated with epithermal deposits.

There is also a particularly close association between dome margins, dikes, and epithermal vein

8 Calderas & Flow Domes (David M. Jones, 2005) Large-scale calderas with medium- to high-level intrusions including andesitic to rhyodacitic flow domes overlain by thick ignimbrite units are associated with epithermal deposits. Mineralization is intimately related to intra-graben regional strike-slip shearing & caldera magmatism. There is also a particularly close association between dome margins, dikes, and epithermal vein mineralization. Contorted flow banding & sheeting at margin of rhyodacite dome. David Jones explaining caldera-setting with dome feature in background. Sub-horizontal mushroom- lobe flow banding in felsite dike. 8

Strike-Slip Shearing & Magmatism Aguila 16 km Arista Megabreccia with clasts of mainly andesite define a paleosurface contemporaneous with mineralization.

9 Strike-Slip Shearing & Magmatism Aguila 16 km Arista Megabreccia with clasts of mainly andesite define a paleosurface contemporaneous with mineralization. Caldera I Resurgent Dome Caldera II Local magmatism begins with Caldera I formation denoted by deposition of densely welded intracaldera tuff Caldera I formation followed by resurgent dome with small-volume pyroclastics erupted onto a paleosurface Caldera II development involves late piston-like exaggerated uplift of Cretaceous basement & deposition of pyroclastic apron deposits around and along margins of this uplift N70W Regional Structure Piston-Like Plug Resurgent Domes Nested Calderas

Paleosurface Deposits Highest priority exploration targets lie along & below the

Important paleosurface indicators: Travertine deposited from carbonate-bearing water

Arista, carbonated subsurface water bubbling with CO 2 gas and temperatures 42 43

10 Paleosurface Deposits Highest priority exploration targets lie along & below the paleosurface where it intersects annular structures related to resurgence. Important paleosurface indicators: Travertine deposited from carbonate-bearing water and indicative of a still active hydrothermal system (Above & upper right); At Arista, carbonated subsurface water bubbling with CO 2 gas and temperatures degrees centigrade at 400 m depth Paleosurface fossils like this bark and twig impression (Lower right) 10

meters Low & Intermediate sulfidation systems 400 to 1,000 m below

11 Arista Deposit Arista deposit is main epithermal vein system at Aguila mine Drilling has defined vein system to a depth of more than 600 meters Low & Intermediate sulfidation systems 400 to 1,000 m below paleosurfaces Arista deposit is a intermediate sulfidation epithermal vein system 11

12 Arista Vein Polymetallic mineralization mainly comprises: pyrite sphalerite galena chalcopyrite argentite/acanthite pyrargyrite tetrahedritetennanite arsenopyrite stibnite Fluid Inclusions (quartz): C <2% wt% NaCl 12

13 Skarn Association (Jeffrey A. Jaacks, 2006) Before discovery of Arista, studies already showed that precious & base metals introduced in 2 separate events: early skarn followed by epithermal Geochemical sampling studies indicated 2 different metal associations: Au+Ag+As+Sb+Hg+Cu+Pb+Zn+Mo(+Bi+W): representing Au-Ag-Base Metal veins developed in skarn setting peripheral to an intrusive, and Au+Ag+As+Sb+Hg: a more limited element suite, more typical of a volcanic hosted epithermal Au-Ag vein system 13

quartz-galena and tan bustamite (CaMn 2+

(Sphalerite Fluid Inclusions: 260-300 C;

14 Skarn Alteration (Lawrence D. Meinert, 2010) Multiple generations of quartz-galena and tan bustamite (CaMn 2+ Si 2 O 6 [1] ) Quartz vein with white fibrous wollastonite Banded green pyroxenesphalerite-galena-chalcopyrite (Sphalerite Fluid Inclusions: C; 6-16% wt% NaCl) Dolomitic limestone cut by calcite-talc-tremolite veins 14

Magnetics & Intrusives Interpretation of magnetic data by geophysicist, Bob Ellis, using standard digital image processing techniques & inversion

15 Magnetics & Intrusives Interpretation of magnetic data by geophysicist, Bob Ellis, using standard digital image processing techniques & inversion modeling, helped extend known mineralized structures & identify areas of potential magnetite destructive alteration & skarn mineralization Caldera I 15

16 3D Magnetics & Geology Integrating 3D modeling with geology has been helpful in targeting at the mine scale & provides better understanding of the regional geology Mag low associated with Arista deposit & mag highs mixed intrusives & skarn 16

Exploration Model (Corbett, 2006) Extension-related pull-apart basin model for proposed Taviche Graben and related veins/skarn Controlling strike-slip

17 Exploration Model (Corbett, 2006) Extension-related pull-apart basin model for proposed Taviche Graben and related veins/skarn Controlling strike-slip structure creates pull-apart basin into which were deposited Miocene volcanics with hot spring & sinter deposits & steep, en echelon fissure veins & intrusions/skarn

18 OAXACA Gold & Silver Production GOLD (oz) MINE TOTALS AGUILA (Gold Resource) SAN JOSE (Fortuna Silver) SILVER (oz) 10,493 21,586 34,417 33,942 35,552 29, ,634 4,524 17,918 19,031 33,496 38, ,495 TOTALS 10,493 26,110 52,335 52,973 69,048 68, ,129 MINE TOTALS AGUILA (Gold Resource) SAN JOSE (Fortuna Silver) 111,316 2,180,309 2,996,743 3,032,841 3,297,204 2,506,000 14,124, ,167 1,949,178 2,527,203 4,396,760 4,928,893 14,280,201 TOTALS 111,316 2,658,476 4,945,921 5,560,044 7,693,964 7,434,893 28,404,614 Increasing reserves at both operations indicate more years of production are ahead 18

Calderas. Myojin Knoll Submarine Caldera m. 500 m. 5 km. (after Kennedy and Stix, 2003)

Calderas. Myojin Knoll Submarine Caldera m. 500 m. 5 km. (after Kennedy and Stix, 2003) Calderas Myojin Knoll Submarine Caldera 1400 m 500 m 5 km (after Kennedy and Stix, 2003) Definition Outline Relationships to Eruption Volume and VEI Structural Components Types Caldera Genetic Models and